Diamond coated cutting tools demonstrate excellent metalcutting properties in certain circumstances. Typically, diamond coated cutting tools comprise a substrate of cemented (cobalt) tungsten carbide that has a diamond coating on the surface thereof. It has always been, and still remains, a goal to improve the adhesion of the diamond coating to the substrate.
U.S. Pat. No. 5,585,176 to Grab et al. and U.S. Pat. No. 5,648,119 to Grab et al. each shows a diamond coated cutting tool. In the process to make the substrate for the cutting tool of the '176 Patent and the '119 Patent, there is a sintering step (or re-sintering step) that is supposed to cause grain growth and cobalt (i.e., binder) depletion at the surface of the substrate. Larger grains at the surface, as well as the depletion of cobalt binder at the surface, generally improve the adhesion of the diamond coating to the substrate.
While the process of the '176 Patent and the '119 Patent produce diamond coated cutting tools with acceptable adhesion properties, the re-sintering process is sensitive to the level of carbon in the cobalt binder of the substrate prior to re-sintering. The magnetic saturation value is a measure of the carbon content in the cobalt binder. Magnetic saturation is generally reported in either microtesla cubic meter per kilogram cobalt (μT-m3/kg) or gauss cubic centimeter per gram cobalt (gauss-cm3/gm). When the magnetic saturation is too low the cobalt binder is not sufficiently mobile (or fluid), and as a result, the cobalt does not evaporate from the surface of the substrate during re-sintering. When the magnetic saturation is too high, a cobalt binder-carbon cap forms on the surface of the substrate during re-sintering that effectively halts the continued evaporation of the cobalt binder.
Heretofore, in the production of commercial quantities of the re-sintered cutting tool substrates the magnetic saturation value of the sintered substrates, i.e., the substrate prior to re-sintering, must fall within a narrow range. Practically speaking, such a narrow range for the magnetic saturation value is difficult to accurately measure. This difficulty in measuring may result in not all of the sintered substrates falling within the prescribed range of the magnetic saturation value which may, in turn, result in re-sintered substrates that have certain drawbacks as described above if the magnetic saturation value is either too high or too low. Iron contamination can also affect the magnetic saturation value by causing it to be over reported. This may also result in the re-sintering of sintered substrates that do not have a magnetic saturation value within the prescribed range.
These drawbacks associated with the difficulty in accurately measuring the magnetic saturation value make it highly desirable to provide a process for making a diamond coated cutting tool that does not exhibit this sensitivity to the carbon content in the cobalt binder of the sintered substrate, and hence, accommodates a sintered substrate with a broader range of magnetic saturation values. In other words, it would be highly desirable to provide a process for making a resultant cutting tool substrate (and provide the resultant substrate itself) that is suitable for diamond coating and that accommodates a broader range of acceptable magnetic saturation values of the sintered substrate so as to have a broadened so-called “carbon window” as compared to earlier processes.
Heretofore, in the production of diamond coated cutting tools that use a re-sintered cutting tool substrate, the grain size of the tungsten carbide in the sintered substrate has been fine. As a result of using the fine-grained sintered substrate the extent of re-sintering has had to be relatively long to achieve tungsten carbide grains with a sufficiently larger grain size in the re-sintered substrate. It would be highly desirable to provide a diamond coated cutting tool that comprises a diamond-coated re-sintered substrate wherein the substrate does not require as long a re-sintering time to produce a re-sintered substrate that has tungsten carbide that is of a sufficiently large grain size.